It is known to use chromatic confocal techniques in optical height or distance sensors. As described in U.S. Pat. No. 7,876,456 (the '456 patent), which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the surface height or distance relative to the focusing element determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. Upon reflection from the surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the aperture. All of the other wavelengths are poorly focused on the aperture and so will not couple much power into the fiber. Therefore, for the light returned through the fiber, the signal level will be greatest for the wavelength corresponding to the surface height or distance to the surface. A spectrometer type detector measures the signal level for each wavelength in order to determine the surface height.
The '456 patent further describes that certain manufacturers refer to practical and compact systems that operate as described above, and that are suitable for chromatic confocal ranging in an industrial setting, as chromatic point sensors (CPS). A compact, chromatically-dispersive optical assembly that is used with such systems is referred to as an “optical pen,” or a “pen.” The optical pen is connected through an optical fiber to an electronic portion of the chromatic point sensor which transmits light through the fiber to be output from the optical pen and which provides a spectrometer that detects and analyzes the returned light. The returned light forms a wavelength-dispersed intensity profile received by the spectrometer's detector array. Pixel data corresponding to the wavelength-dispersed intensity profile is analyzed to determine the “dominant wavelength position coordinate” as indicated by a peak or centroid of the intensity profile, and the resulting pixel coordinate of the peak or centroid is used with a lookup table to determine the distance to the surface. This pixel coordinate may be determined with sub-pixel resolution and may be referred to as the “distance-indicating coordinate.”
The '456 patent further describes that, in normal operation, the CPS spectrometer ordinarily receives a certain range or peak region of wavelengths for a certain measurement distance. It is disclosed that the CPS spectrometer may distort the shape of peak region of wavelengths, and thus influence the corresponding peak or centroid and the resulting distance-indicating coordinate. The systems and methods of the '456 patent provide component calibration data, also referred to as compensation data, that encompasses the effects of wavelength-dependent variations (e.g., non-uniform response) in the CPS spectrometer and/or the CPS broadband light source. The compensation data of the '456 patent is used to reduce or eliminate errors associated with these effects in the spectrometer and light source. The compensation data of the '456 patent may be redetermined and/or replaced at various points in time, such that the compensation data remains effective to reduce or eliminate errors despite changes in the spectrometer and/or light source characteristics (e.g., due to component aging, environmental variations, or the like).
Chromatic point sensors provide very high resolution and accuracy (e.g., sub-micron resolution and accuracy) based on distance calibration data that correlates known measurement distances with the resulting dominant wavelength position coordinate (the distance-indicating coordinate) along the array. At the level of resolution and accuracy provided by chromatic point sensors, measurement errors may occur as a result of measurement conditions failing to precisely match the conditions present at the time of calibration, even when the methods of the '456 patent are used.
For example, one source of error in a chromatic point sensor is workpiece-specific spectral reflectivity variations. A chromatic point sensor which allegedly provides a means for compensating for spectral reflectivity variations is disclosed in U.S. Pat. No. 5,790,242 (the '242 patent). Briefly, a confocal beam is split, and the split portions of the beam are spatially filtered at their focus with a confocal aperture and a central stop (which is “an inverse pinhole”) to provide respective signals on respective energy collecting detectors. The '242 patent describes that “at an instant of time within the sweep” when the ratio between the aperture signal and the signal from the energy that passes around the central stop is at a maximum, the energy through the aperture is the result of “focus,” independent of the reflectivity of the target. A calibration is provided (by unspecified means) which relates the wavelength at this instant in time to the depth of the surface. However, the accuracy of the '242 patent therefore depends on providing a “sweep,” and on sensing a relationship between two signals “at an instant,” and detecting a dominant wavelength in one of the signals at that instant. Such a system introduces electronic complexity and associated noise sources, and may be difficult for a user to comprehend and/or calibrate. Furthermore, such a system does not gather any data characterizing the target surface reflectivity characteristics, which may be desired as an accuracy or calibration verification, or a material verification, in various applications.
Providing improved and/or more reliable operation for chromatic point sensors by overcoming additional sources of measurement errors that result from changing measurement conditions, and particularly from workpiece-specific spectral reflectivity variations, is desirable.